Oil resistant rubber-like material containing a butadiene-acrylonitrile copolymer and calcined magnesia



OIL RESISTANT RUBBER-LIKE MATERIAL CON- TAINING ABUTADIENE-ACRYLONITRILE CO- POLYMER AND CALCINED MAGNESIA No Drawing.Application June 6, 1956 Serial No. 589,602

17 Claims. (Cl. 26023.7)

This invention relates to an oil-resistant rubber-like material, and toarticles made therefrom, which is also resistant to breakdown at atleast moderately high temperatures. More particularly the inventionrelates to a material of the indicated type which may be molded, orotherwise shaped, and vulcanized.

In many of the larger present day airplanes the control surfaces andengine controls are remotely operated by hydraulic devices. Duringservicing of the airplanes, and even during their operation, at leastslight amounts of the oil or oil-like hydraulic fluid in the hydraulicmotors and in the tubing connected thereto tend to leak and thus tocover the surfaces of adjacent equipment. Such fluid has a particularlydeleterious efiect upon natural rubber and the ordinary syntheticrubbers, with which electrical wires and similar equipment may hecovered, particularly at elevated temperatures such as are encounteredin engine compartments, engine fire walls, and the like.

The rubber-like material of the present invention is particularlyresistant to the action of oils or oil-like fluids such as hydrauliccontrol fluid both at atmospheric and at moderately high temperatures.The material of the invention is readily molded, or otherwise shaped,and vulcanized, and canbe madeinto a great variety of shapesbysubstantially conventional procedures. Thus the material can be moldedin dies to form articles such as grommets to carry electrical wiringthrough metal supports such as engine partitions, fire walls, and thelike, or it may be extruded in the form of insulating sheaths on wiresor flexible tubing. 7

The invention has among its objects the provision of an electricallyinsulating rubber-like material which sustains heating to at leastmoderately elevated temperatures for long periods without undue changein its hardness. 'The invention also includes shaped and vulcanizedarticles made from such material.

A further object of the invention is the provision of an electricallyinsulating rubber-like material which sustains heating to elevatedtemperatures for long periods while immersed in oil or oil-like liquidswithout undue change in its hardness, undue change in its volume, orundue loss-in tensile strength.

Yet another object of the invention resides in the provision of anelectrically insulating rubber-like material of the character indicatedwhich has good tensile strength, high percentage elongation, andadequate hardness.

A still further object of the invention resides in the provision of anoil and high temperature resistant elec- States Patent tricallyinsulating rub-ber like -material which has high i molds, and for beingvulcanized, without dimensional variations beyond existing tolerances,which is. highly resistant to cracking when bent at low temperatures,and which does not acquire an undue set when compressed for long periodsat high temperatures.

A still further object of the invention is the provision of anoil-resistant electrically insulating rubber-like mate.- rial of. thecharacter indicated which may readily be extruded and which, whenvulcanized, has high chaferesistant properties. ,I

The above and further objects of the invention will become apparent uponconsideration of the following specification setting forth preferredembodiments of the composition of the material, preferred methods ofcompounding the material, and some permissible variations in suchcompositions and methods of compounding and processing them.

The oil-resistant electrically insulating rubber-like material of theinvention is' composed primarily of a synthetic rubber-like basecompound of a butadiene-acrylonitrile copolymer and a large amount offinely divided magnesium oxide dispersed substantially uniformlythroughout the base compound. To such mixture there are added small buteffective amounts of other ingredients which function to give theresulting material specific desired properties upon curing andvulcanizing, and to provide in the final articles molded or shapedtherefrom desired characteristics under arduous service conditions. Thefollowing table gives the permissible range of variation of a firstembodiment of the composition of the invention:

Embodiment 1 l Parts by weight Hycar #1001 (synthetic rubber base) Mg 0(heat resistant filler) 100-125 Zn 0 (accelerator activator) 5-15Stearic acid (activator) 0.5-1.5 Methyl Tuads (primary ultraaccelerator) (and vulcanizing agent) 1.5-4.0 Sulfasan R (primaryaccelerator) (and/or curing agent) 1.0-2.5 Thermoflex A (anti-oxidant)2-10 Aminox (anti-oxidant). 2-10 Sunproof Jr. (inhibitor of exposuredamage) 3-10 Esen (retarder) 0.25-l.5

I-Iycar #1001 is the trade name of a butadiene-acrylonitrile copolyrnerhaving a high acrylonitrile content. The Mg 0 employed is finely dividedextra light calcined magnesia. The Zn 0 is finely divided zinc oxide ofthe grade known as zinc white or Chinese white.

The stearic acid, also known as octadeconoic acid, is

likewise in finely divided form.

Methyl Tuads is a trade name for tetramethylthiuram disulfide. SulfasanR is a trade name for morpholine disulfide. Thermoflex A is" the tradename of a substituted aromatic amine consisting of:

50% phenyl-beta-naphthylamine- 25% di-para-methoxydiphenylamine 25%diphcnyl-para-phenylencdiamine Arninox is the trade name of a lowtemperature reaction product of diphenylamine and acetone. Sunproof Jr.is the trade name of a specially selected mixture of waxy hydrocarbons;such material is also known as Heliozone and Sunolite. Esen is a tradename of phthalic anhydridel The functions of the ingredients aregenerally indicated in the first table above. More specifically, MethylTuads functions as a primary ultra accelerator and vulcanizing agent. Asa vulcanizing agent it permits the attainment of far superior heat-agingand compression set properti'es than conventional agents.

Sulfasan R acts as a primary accelerator and/or ouring agent, and alsofunctions to inhibit scorching of the material. It also imparts superioraging properties in the above material. ThermoflexA acts as anantioxidant, as does also the Aminox. Sunproof Jr. protects thevulcanized material from deterioration due to exposure, inhibitscracking of the material by ozone, and also functions as ananti-frosting agent. Esen acts to prevent scorching of the material atprocessing temperatures.

It will be seen from the above that zinc oxide and stearic acid bothfunction as an accelerator and activator, that Methyl Tuads and SulfasanR both function as a primary accelerator, and that both Thermofiex A andAminox function as an antioxidant. Methyl Tuads also has the furtherfunction of vulcanizing agent, Sulfasan R has thefurther function ofinhibiting the scorching of the material, as does Esen.

From the standpoint of function, the composition set out in the firsttable may be partially retabulated as follows:

Parts by Total weight Accelerator and Activator:

Stearie acid 0.5-1.5 2. Primary accelerato Methyl Tuads--- 1. 5-4.0 HI 5Sulfasan R 3. Anti-oxidant:

Therrnoflex A 2-10 440 Aminox 210 4. Vulcanizing Agent:

See (2) Methyl Tuads.. 1.54.0 5. Curing Agent:

See (2) Sulfasan R 1. 02. 5 0. Exposure Damage Inhibitor:

Snnproof Jr 3-10 7. Alrtiscorching Agent' 0 25 1 5 sen See (2 and (5Sulfasan R 1.0-2. 5 i

Variation of the composition within the above defined ranges providesproperty characteristics within a prescribed desired range of limits. Acomposition lying within the above ranges, and having characteristicspresently preferred for the making of grommets, is that given in thefollowing example.

A preferred precedure of making the material set forth in the abovetables is as follows:

The Hycar is first broken down on a tight, cold mill, with rolltemperature between 90 and 110 F. The zinc oxide is added first, and isthoroughly milled and blended into the base compound. The Thermoflex A,Aminox, and Esen are then added and thoroughly blended into the basecompound.

Then, in that order, the Sulfasan R is added and blended, a part,preferably one-third, of the magnesia is added and thoroughly blended, apart, preferably onehalf, of the stearic acid is added and blended,one-third of the magnesia and all the Sunproof Jr. are added and blendedthoroughly, the remainder of the magnesia is added and blendedthoroughly, and the remainder of the stearic acid is added and blendedthoroughly.

The compound is then sheeted off, and allowed to cool and age for atleast 24 hours. After this, the stock is warmed up on a cool mill, andMethyl Tuads are added and thoroughly blended.

Care should be taken to keep the mill roll temperatures as low aspossible during the entire mixing and Warm-up periods.

After the above-described addition of the Methyl Tuads, the compound isready for the formation of articles by conventional molding andvulcanizing procedures.

Test samples made by molding and vulcanizing stock material prepared inthe manner set forth above and having the composition set out in thetable of Example 1 had the following initial properties:

Tensile strength 2200 psi min.

Elongation 600 min.

Shore hardness 72-75 Dielectric strength 500 volts per mil.

The samples withstood continuous exposure to 10 parts per million ozoneWithout checking for 5 minutes.

The samples possessed excellent resistance to aging at elevatedtemperatures. Thus, grommets heated to 250 F. for 70 hours and thencooled changed in hardness a maximum of 15 points on the Shore Ahardness scale, and their sidewalls underwent a 180 flat bend withoutcracking.

Such grommets successfully withstood a very exacting test involving oilimmersion at elevated temperature. In such test, the grommets wereimmersed in an oil designated in the trade as Esso Turbo #15 for aperiod of 70 hours at 300 F. After such heating, the grommets werewithdrawn from the oil bath, cooled, and cleaned. The hardness change ofthe thus treated grommets was no greater than :10 points on the Shore Ascale, the volume of the grommets had changed no more than -5% to 30%,and the tensile strength of their material was no less than 1000lbs./in.

Embodiment 2 Partsb weight Plastlclzer (base on 100 Total partsolHycar)'rP-es 10-20 1H5 Tricresyl phosphate 5-15 TP- is a trade name of thehigh molecular weight polyether ester, dibutoxy ethoxy ethyl adipate.TP-95, which functions in the compound of the second embodiment as a lowtemperature plasticizer and softener, has no effect on the curing of thematerial. It imparts excellent low temperature flexibility to thevulcanized product, combined with high-temperature resistance.

Tricresyl phosphate functions as a low temperature plasticizer,softener, and flame retarder. It aids in processing the material,imparts low-temperature flexibility, and gives the material fireproofingqualities. Because of its low water solubility, TP-95 aids in giving thematerial moisture resistance.

A composition lying Within the above indicated ranges of variation ofthe ingredients (the first table with the above-plasticizer added), andhaving characteristics presently preferred for the making of the outersheathing 'aessaee layer of a Flexible hose having marked anti-chafingproperties, is that given in the following example.

Example 2 Parts by weight Hycar #1001 100 Zn Stearic acid 1 Extra lightcalcined Mg 0 100 Thermoflex A 5 Aminox 5 Methyl Tuads 1.50 Sunproof Jr.i 5 Esen 1 Sulfasan R 1.50 TP-95 l0 Tricresyl phosphate The material ofthe second embodiment of the invention is preferably processed in thesame manner as the first embodiment, with the exception that theplasticizers are added slowly after all of the magnesia has been addedand blended. After this, the compound is sheeted 0E and allowed to cooland age for at least 24 hours, following which the stock is warmed up ona cool mill, and'the Methyl Tuads is added and thoroughly blended, as inthe first embodiment.

The thus prepared material is used to sheath hoses or cables byconventional practices, as by continuously extruding the material aboutthe central portion of the hose or cable acting as a core, after whichthe sheathed product is vulcanized in accordance with conventionalprocedure.

The sheath of a hose made in the above manner and having the compositionset out in the table of Example 2 had properties which were not markedlydifierent from those of the above described grommets.

it is to be understood that within the teaching of the invention somevariations are possible. The following permissible substitutes mayreplace the indicated ingredient part for part.

For Hycar #1001 there may be substituted the somewhat similarbutadiene-acrylonitrile copolymers designated in the trade as ParacrilD, Chemigum N, Butaprene NXM, and Hycar #1041 or'a mixturethereof.

For stearic acid there may be substituted zinc stearate, oleic acid, orthe material designated in the trade as Laurex.

For extra light calcined magnesia there may be substituted any regularNeoprene grade of magnesia.

For Thermoflex A there may be substituted U.O.P. 88, U.O.P. #288,Tenamene #2, Tenamene #30, Aminox, or Agerite Resin D. i v

For Aminox there may be substituted Thermofiex A or the other materialsset forth in the preceding paragraph.

For Methyl Tuads there may be substituted Ethyl Tuads, Sulfasan R,or'Dicumyl peroxide.

For Sunproof Ir. there may be substituted any microcrystalline wax orany other waxy hydrocarbons used in rubber compounding. Among these areSunolite Wax, Crown Wax, paraifin, Carnauba, and Ceresins.

For Esen there may be substituted Vultrol,-Retarder W, or salicyclicacid.

For Sulfasan R there may be substituted Tetrone A Dicumyl peroxide, orMethyl Tuads. I

For TP-95 there may be substituted any low temperature plasticizer thatis compatible with Buna N polymers and imparts flexibility to thecompound at 65 F. One such plasticizer is Plasticizer SC.

For tricresyl phosphate there may be substituted TP- 95, dioctylphthalate, Paraplex AL-lll, Paraplex G-25, or ilasticizer A-l 18.

Also, in accordance with the invention, the compositions of Embodiments1 and 2 may be varied by adding 5-10 parts of U.O.P. 88, U.O.P. #288, orone of the Tenamenes. Thermofiex A and Aminox may be omitted andreplaced part by part with U.O.P. 88, U.O.P. #288, one of the Tenamenes,or Agerite Resin D. From 2 to 10 parts of a low molecular weightpolyethylene may be added. Esen may be omitted. From 2 to 5 parts ofAgerite Resin D may be added. Up to 5 parts of iron oxide may be added.

Although for purposes of illustration we have disclosed preferredembodiments and examples of the oilresistant rubber-like material of ourinvention, and have disclosed some variations thereof, it will beunderstood that such embodiments and examples are illustrative only, andthat the invention is defined by the claims appended hereto. 1

We claim:

1. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising about 100 parts by weight of a base compoundconsisting essentially of butadieneacrylonitrile copolymer, from about100 to 125 parts by weight of finely divided calcined magnesia, andsmall but effective amounts of at least one vulcanization acceleratorand activator.

2. A substantially homogeneous molded and vulcanized electricallyinsulating article made of oil-resistant rubberill-6 material comprisinga base compound consisting essentially of 100 parts by weight ofbutadiene-acrylonitrile copolymer, from about 100 to 125 parts by weightof finely divided calcined magnesia, and small but effective amounts ofa vulcanization activator and a vulcanization accelerator.

3. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial consisting essentially of about 100 parts of a base compound ofbutadiene-acrylonitrile copolymer, from about 100 to 125 parts of finelydivided calcined magnesia, from 5.5 to 16.5 parts of a vulcanizationactivator, and from 2.5 to 6.5 parts of a vulcanization accelerator, allparts being by weight.-

4. A vulcanizable oil-resistant rnbberdike electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts by Weight of butadieneacrylonitrile' copolymer, from about 100 to125 parts by weight of finely divided calcined magnesia, and small buteffective amounts of at least one vulcanization accelerator andactivator, an anti-oxidant for the material, a vulcanizing agent, amaterial curing agent, a vulcanization retarder, and an agent inhibitingexposure damage to the material.

5. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of aboutparts of buta'diene-acrylonitrile copolymer, from about 100 to parts offinely divided calcined magnesia, from 5.5 to 16.5 parts of avulcanization activator, from 2.5 to 6.5 parts of a vulcanizationaccelerator, from 4 to 20 parts of an anti-oxidant for the materiah'from1.5 to 4.0 parts of a vulcanizing agent, from 1.0 to 2.5 parts of amaterial curing agent, from .25 to 1.5 parts of a vulcanizationretarder, and from 3 to 10 parts of an agent inhibiting exposure damageto the material, all parts being by weight.

6. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essential of about 100parts of butadiene-acrylonitrile copolymer, from about 100 to 125 partsof finely divided calcined, magnesia, from 5.5 to 16.5 parts of avulcanization activator composed of zinc oxide and stearic acid, andfrom 2.5 to 6.5 parts of a vulcanization accelerator composed oftetramethylthiuram disulfide and morpholine disulfide, all parts beingby weight.

7. A substantially homogeneous molded and vulcanized electricallyinsulating article made of oil-resistant rubberlilce material comprisinga base compound consisting essentially of about 100 parts ofbutadiene-acrylonitrile copolymer, from about 100 to 125 parts of finelydivided calcined magnesia, from 5.5 to 16.5 parts of a vulcanizationactivator composed of zinc oxide and stearic acid,

a 7 and from 2.5 to 6.5 parts of a vulcanization accelerator composed oftetramethylthiuram disulfide and morpholine disulfide, all parts beingby weight.

8. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts of butadiene-acrylonitrile copolymer, from about 100 to 125 partsof finely divided calcined magnesia, from 5.5 to 16.5 parts of avulcanization activator, from 2.5 to 6.5 parts of a vulcanizationaccelerator, from 4 to 20 parts of an antioxidant for the material, from1.5 to 4.0 parts of a vulcanizing agent, from 1.0 to 2.5 parts of amaterial curing agent, from .25 to 1.5 parts of a vulcanizationretarder, and from 3 to 10 parts of an agent inhibiting exposure damageto the material, all parts being by weight.

9. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts of butadiene-acrylonitrile copolymer, from about 100 to 125 partsof finely divided calcined magnesia, from 5.5 to 16.5 parts of avulcanization activator, from 2.5 to 6.5 parts of a pri maryvulcanization accelerator, minor but effective amounts of ananti-oxidant for the material composed of a substituted aromatic amineand a low temperature reaction product of diphenylamine and acetone, ofa vulcanizing agent composed of tetramethylthiuram disulfide, a materialcuring agent composed of morpholine disulfide, a vulcanization retardercomposed of phthalic anhydride, and a mixture of waxy hydrocarbonsinhibiting exposure damage to the material, all parts being by Weight.

10. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts of butadiene-acrylonitrile copolymer, from about 100 to 125 partsof finely divided calcined magnesia, from 5.5 to 16.5 parts of avulcanization activator, from 2.5 to 6.5 parts of a primaryvulcanization accelerator, from 4 to 20 parts of an anti-oxidant for thematerial composed of a substituted aromatic amine and a low temperaturereaction product of diphenylamine and acetone, from 1.5 to 4.0 parts ofa vulcanizing agent composed of tetramethylthiuram disulfide, from 1.0to 2.5 parts of a material curing agent composed of morpholinedisulfide, from .25 to 1.5 parts of a vulcanization retarder composed ofphthalic anhydride, and from 3 to 10 parts of a mixture of waxyhydrocarbons inhibiting exposure damage to the material, all parts beingby weight. 7

11. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts of a base compound of butadiene-acrylonitrile copolymer, fromabout 100 to 125 parts of finely divided calcined magnesia, from 5.5 to16.5 parts of a vulcanization activator composed of zinc oxide andstearic acid, from 2.5 to 6.5 parts of a vulcanization acceleratorcomprising tetramethylthiuram disulfide, from 4 to 20 parts of ananti-oxidant for the material composed of a substituted aromatic amineand a low temperature reaction product of diphenylamine and acetone,from 1.5 to 4.0 parts of a vulcanizing agent composed oftetramethylthiuram disulfide, from 1.0 to 2.5 parts of a material curingagent composed of morpholine disulfide, from .25 to 1.5 parts of avulcanization retarder composed of phthalic anhydride, and from 3 to 10parts of a mixture of waxy hydrocarbons inhibiting exposure damage tothe material, all parts being by weight.

12. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial consisting essentially of about 100 parts of a base compound ofbutadiene-acrylonitrile copolymer, from about 100 to 125 parts of finelydivided calcined magnesia, from to 15 parts of zinc oxide, from 0.5 to1.5 parts of stearic acid, from 1.5 to 4.0 parts of tetramethylthiuramdisulfide, from 1.0 to 2.5 parts 50% phenyl beta-naphthylamine 25%di-para-methoxydiphenylamine 25% diphenyl-para-phenylenediamine minorbut effective amounts of a low temperature reaction product ofdiphenylamine and acetone, an inhibitor of exposure damage composed of amixture of waxy hydrocarbons, and phthalic anhydride, all parts being byweight.

13. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial consisting essentially of about parts of a base compound ofbutadiene-acrylonitrile copolymer, from about 100 to parts of finelydivided calcined magnesia, from 5 to 15 parts of zinc oxide, from 0.5 to1.5 parts of stearic acid, from 1.5 to 4.0 parts of tetramethylthiuramdisulfide, from 1.0 to 2.5 parts of morpholine disulfide, from 2 to 10parts of a mixture composed of 50% phenyl-beta-naphthylamine 25di-para-methoxydiphenylamine 25% diphenyl-para-phenylenediamine from 2to 10 parts of a low temperature reaction product of diphenylamine andacetone, from 3 to 10 parts of an inhibitor of exposure damage composedof a mixture of waxy hydrocarbons, and from .25 to 1.5 parts of phthalicanhydride, all parts being by weight.

14. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts by weight of butadiene-acrylonitrile copolymer, from about 100 to125 parts by weight of finely divided calcined magnesia, a minor amountof at least one low temperature plasticizing agent for the material, andsmall but effective amounts of at least one vulcanization acceleratorand activator.

15. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial comprising a base compound consisting essentially of about 100parts of butadiene-acrylonitrile copolymer, from about 100 to 125 partsof finely divided calcined magnesia, from 15 to 35 parts of aplasticizer for the material composed of dibutoxy ethoxy ethyl adipateand tricresyl phosphate, from 5.5 to 16.5 parts of a vulcanizationactivator, and from 2.5 to 6.5 parts of a vulcanization accelerator, allparts being by weight.

16. A vulcanizable oil-resistant rubber-like electrically insulatingmaterial consisting essentially of about 100 parts of a base compound ofbutadiene-acrylonitrilc copolymer, from about 100 to 125 parts of finelydivided calcined magnesia, from 5 to 15 parts of zinc oxide, from 0.5 to1.5 parts of stearic acid, from 1.5 to 4.0 parts of tetramethylthiuramdisulfide, from 1.0 to 2.5 parts of morpholine disulfide, from 2 to 10parts of a mixture composed of 50% phenyl-beta-naphthylamine 25di-para-methoxydiphenylamine 25 diphenyl-para-phenylenediamine from 2 to10 parts of a low temperature reaction product of diphenylamine andacetone, from 3 to 10 parts of an inhibitor of exposure damage composedof a mixture of Waxy hydrocarbons, from .25 to 1.5 parts of phthalicanhydride, and a plasticizing agent for the material composed of from 10to 20 parts of dibutoxy ethoxy ethyl adipate and from 5 to 15 parts oftricresyl phosphate, all parts being by Weight.

17. A vulcanizable oil-resistant rubber-like material consistingessentially of a base compound of butadieneacrylonitrile copolymer,about an equal amount by Weight of finely divided calcined magnesia, andsmall but effective amounts of at least one vulcanization acceleratorand activator.

(References on following page) 10 References Cited in the file of thispatent Synthetic Rubber, by Whitby, pp. 808-15, John Wi- E T ley, NewYork, NY. (1954). (Copy in Scie. Libr.)

UNIT D STA Es PATENTS Hycar Synthetic Rubber Compd. Manual for Type2343524 Blake 1944 O. R., volume 1, 1941, Hycar Chem. Company, Akron,

2,459,874 Fay Jan. 25, 1949 5 Ohm 2'524424 Buret 1950 Softener Study 24for Type Or-15, Hycar Chem. Com- FOREIGN PATENTS pany, Akron, Ohio.

582,268 Great Britain Nov. 11, 1946 OTHER REFERENCES 10 Trans. Inst.Rug. Ind., 25, pp. 265-80 (1949). (Copy in Scie. Libr.)

1. A VULCANIZABLE OIL-RESISTANT RUBBER-LIKE ELECTRICALLY INSULATINGMATERIAL COMPRISING ABOUT 100 PARTS BY WEIGHT OF A BASE COMPOUNDCONSISTING ESSENTIALLY OF BUTADIENEACRYLONITRILE COPOLYMER, FROM ABOUT100 TO 125 PARTS BY WEIGHT OF FINELY DIVIDED CALCINED MAGNESIA, ANDSMALL BUT EFFECTIVE AMOUNTS OF AT LEAST ONE VULCANIZATION ACCELERATORAND ACTIVATOR.